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Assessment of the current state of the near-surface part of the geological environment and understanding of its layered structure play an important role in various scientific and applied fields. The presented work is devoted to the application of phasometric modifications of geoelectric control methods to solve the problem of the detailed complex study of the underground layers of the environment in the process of drilling operations with the use of special equipment. These studies are based on the analysis of variations in phase parameters and characteristics of an artificially excited multiphase electric field to assess poorly distinguishable details and changes in the layered structure of the medium. The proposed method has increased accuracy, sensitivity and noise proofness of measurements, which allows for extracting detailed information about the heterogeneity, composition and stratification of underground geological formations not only in the zone where the drill makes contact with the medium, but also in the entire control zone. This paper considers practical mathematical models of phase images for basic scenarios of drill penetration between the layers of the near-surface part of the geological medium with different characteristics, obtained by means of approximation apparatus based on continuous piecewise linear functions, and also suggests the use of modern machine learning methods for intelligent analysis of its structure. Studying the phase shifts in electrical signals during drilling highlights their value for understanding the dynamics of soil response to the process. The observed signal changes during the drilling cycle reveal in detail the heterogeneity in soil structure and its response to changes caused by drilling. The stability of phase shifts at the last stages of the process indicates a quasi-equilibrium state. The results make a significant contribution to geotechnical science by offering an improved approach to monitoring a layered structure without the need for deep drilling. [ABSTRACT FROM AUTHOR]

Assessment of the current state of the near-surface part of the geological environment and understanding of its layered structure play an important role in various scientific and applied fields. The presented work is devoted to the application of phasometric modifications of geoelectric control methods to solve the problem of the detailed complex study of the underground layers of the environment in the process of drilling operations with the use of special equipment. These studies are based on the analysis of variations in phase parameters and characteristics of an artificially excited multiphase electric field to assess poorly distinguishable details and changes in the layered structure of the medium. The proposed method has increased accuracy, sensitivity and noise proofness of measurements, which allows for extracting detailed information about the heterogeneity, composition and stratification of underground geological formations not only in the zone where the drill makes contact with the medium, but also in the entire control zone. This paper considers practical mathematical models of phase images for basic scenarios of drill penetration between the layers of the near-surface part of the geological medium with different characteristics, obtained by means of approximation apparatus based on continuous piecewise linear functions, and also suggests the use of modern machine learning methods for intelligent analysis of its structure. Studying the phase shifts in electrical signals during drilling highlights their value for understanding the dynamics of soil response to the process. The observed signal changes during the drilling cycle reveal in detail the heterogeneity in soil structure and its response to changes caused by drilling. The stability of phase shifts at the last stages of the process indicates a quasi-equilibrium state. The results make a significant contribution to geotechnical science by offering an improved approach to monitoring a layered structure without the need for deep drilling.

We carried out spectral-induced polarization (SIP) measurements on a set of dried seafloor massive sulfide samples and compared the results with those obtained with the same samples fully saturated with NaCl solution. We find that the conductivity and polarizability are generally high for both dried and saturated samples, i.e., exhibiting phase shifts in the order of 100 s of mrad and imaginary conductivities up to 1 S/m. Depending on the particular sample, the polarizabilities of the dried samples remain as high as for the saturated samples or are slightly reduced. The high polarizability of dried samples and the fact that polarizability cannot be destroyed by drying are significant observations because most of the existing theories to explain the polarization of mineralized rock assume a pore space filled with an electrolyte. We also found that the often-used agar gel is unsuitable for coupling the dried samples to the electrodes because it releases water into the sample. Coupling with plasticine is a feasible alternative because significantly less fluid is absorbed by the sample when it is incorporated into the sample holder. [ABSTRACT FROM AUTHOR]

In order to interpret spectral induced polarization (SIP) data measured in the field for the purpose of mineral exploration, laboratory investigations are necessary that establish relationships between electrical parameters and mineral properties. For massive ores, and in particular for seafloor massive sulfides (SMSs), samples may be difficult to obtain, and it is often not desired to cut cylindrical plugs out of the available hand pieces. We suggest a method to obtain the electrical spectra of hand pieces from measurements on the samples embedded in a non-polarizing medium, in our case quartz sand. As such, destroying potentially precious samples is not necessary. The frequency-dependent phase spectrum of the sample is calculated by dividing the bulk spectrum with a so-called dilution factor, which is obtained from numerical simulation and has a real and constant value. We evaluate the method with a set of SMS samples where conventional cylindrical plugs are available. We can estimate the phase shift maximum of 73% of the samples with a deviation less than 50% from the reference. The estimation quality slightly decreases if the dilution factor is approximated by the volumetric share of the sample. We consider the performance acceptable if the general difficulty to obtain reproducible and representative laboratory measurements for massive sulfides is taken into account. [ABSTRACT FROM AUTHOR]

The long-term experiment for electromagnetic monitoring in Lake Baikal and the adjacent territory is underway. Within this experiment, long-period (36–200 days) variations in the vertical component of electric field are revealed in the water column, which are not caused by any internal sources, but show a close lagged relationship with slow variations in the solar X-ray emission, especially with Rieger variations. The variations in the electric field are interpreted as the manifestation of changeability of the current in the hydrospheric section of the Global Electric Circuit over the Baikal Rift due to the indirect influence of the solar X-ray radiation on the convective component of the current in the atmosphere through slow meteorological processes. [ABSTRACT FROM AUTHOR]

Geysers are among the most fascinating geological features on Earth. Yet, little is still known about their hydrogeological structure at depth. To shed light on the spatial relationships between the vertical conduits and the aquifers feeding them, we conducted a 3D geoelectrical campaign in the Haukadalur hydrothermal field, Iceland. We deployed 24 Iris Fullwavers across the hydrothermal field and inverted resistivity and chargeability measurements. Additionally, we measured temperature variations inside Strokkur and Great Geysir geysers showing temperature fluctuations pointing out the oscillatory behavior that characterizes the geysering cycle of the geysers. By combining a semi‐quantitative temperature distribution of the thermal springs across the hydrothermal field with the inversion of the geoelectrical data, we highlight the control that extensional tectonics have on the distribution of fluids across the hydrothermal field. We also point out the occurrence of a common deep groundwater reservoir feeding the hydrothermal centers. Induced polarization data show that the geysers are fed by sub‐vertical water‐filled fracture zones. The geysers are found at the margins of highly resistive regions where we speculate boiling groundwater and vapor is found. Our proposed model suggests that local waters feeding the main groundwater reservoir downwell from the nearby region and then convect upwards, phase transitioning into vapor at about 200 m depth. From here, fluids flow toward the surface through pipes cutting a highly pressurized and hot system. This study shows to the best of our knowledge the first full 3D tomographic image of a hydrothermal field hosting geysers. Plain Language Summary: Geysers are erupting jets of boiling waters. Despite being one of the most fascinating geological processes on Earth, little is still known about their plumbing system at depth. To investigate the 3D geological structure of the Haukadalur hydrothermal field in Iceland, we injected electrical current into the ground and recorded the voltage across the hydrothermal field caused by such an injection. The measured voltage is proportional to the resistive structure of the hydrothermal field where the roots of the Strokkur and Great Geysir geysers are located. We also lowered thermometers into the geyser's conduits to measure temperature variations during geysering cycles. Our multidisciplinary study has shown the occurrence of a fault system along which the hydrothermal fluids are focused. We believe that the shallow hydrothermal springs are fed by a common reservoir at depth but we could not infer whether the two main geysers share the same reservoir. To the best of our knowledge our study shows the first 3D image of the structure of a geyser field. Key Points: 3D resistive structure of the geyser‐hosting Haukadalur hydrothermal fieldVapor‐saturated boiling reservoirs feeding geysers and hydrothermal pondsFluid flow controlled by local tectonics [ABSTRACT FROM AUTHOR]

Abstract—The geoelectric structure of the junction region between three largest segments of the East European Craton (EEC)–Volga–Uralia, Sarmatia, and Fennoscandia—is studied. We analyze electrical conductivity anomalies within the Orsha depression revealed by synchronous array magnetotelluric (MT) and magnetovariational (MV) crustal soundings. The study is based on the data from deep MT/MV survey acquired in 2018–2020 under the SMOLENSK experiment in the vicinity of the Orsha depression and the results of similar studies carried out since 2007 in the previous KIROVOGRAD experiment east on the slope of the Voronezh crystalline massif (VCM). The data are interpreted in the class of two-dimensional (2D) geoelectrical models constructed on the long latitudinal geotraverses combining the soundings from the SMOLENSK array in the west and KIROVOGRAD array in the east. Interpretation of MT/MV data within the Orsha depression is substantially complicated by intricate interference of deep and shallow responses. The latter are determined by current systems within the conductive sedimentary cover, ~2 km thick at the center of the depression and having an integrated longitudinal conductance of ~1000 S. Shallow quasi 3D effects obscure responses of crustal conductive features in a wide range of periods, challenging their analysis in the scope of 2D interpretation of impedance data. Important role in overcoming these obstacles is played by MV tipper data which are immune to surface distortions. To develop the strategy of joint MT/MV interpretation, we carried out experiments on inverting synthetic data calculated in a series of 2D models simulating possible geoelectric structure of the study region. The numerical results demonstrated the effectiveness of using the REduced Basis OCCam's (REBOCC) algorithm in the two-stage successive 2D-inversion scheme which inverts at the first stage the electric (TE) mode MT/MV data alone, and at the second stage incorporates the magnetic (TM) mode impedance data. This technique reliably identifies deep conductive structures beneath thick sedimentary strata. The importance of using a priori information about the geoelectric structure of sedimentary cover when constructing the starting inversion model is demonstrated. The joint 2D inversion strategy worked out on the synthetic MT/MV data was used to interpret observations along three combined profiles of the SMOLENSK and KIROVOGRAD arrays. The interpretation refined the structure of the Kirov–Baryatino crustal anomaly on VCM western slope, revealed complex architecture of crustal anomalies beneath the Orsha depression, and allowed preliminary conclusions about the nature of conductivity anomalies and their connection with ancient and recent geodynamic processes. [ABSTRACT FROM AUTHOR]

We carried out spectral-induced polarization (SIP) measurements on a set of dried seafloor massive sulfide samples and compared the results with those obtained with the same samples fully saturated with NaCl solution. We find that the conductivity and polarizability are generally high for both dried and saturated samples, i.e., exhibiting phase shifts in the order of 100 s of mrad and imaginary conductivities up to 1 S/m. Depending on the particular sample, the polarizabilities of the dried samples remain as high as for the saturated samples or are slightly reduced. The high polarizability of dried samples and the fact that polarizability cannot be destroyed by drying are significant observations because most of the existing theories to explain the polarization of mineralized rock assume a pore space filled with an electrolyte. We also found that the often-used agar gel is unsuitable for coupling the dried samples to the electrodes because it releases water into the sample. Coupling with plasticine is a feasible alternative because significantly less fluid is absorbed by the sample when it is incorporated into the sample holder.

The presence of groundwater is strongly related to its geological and geohydrological conditions. It is, however, important to study the groundwater potential in an area before it is utilized to provide clean water. Werner-Schlumberger's method was used to analyze the groundwater potential while hydraulic properties such as soil porosity and hydraulic conductivity were used to determine the quality and ability of the soil to allow water's movement in the aquifer. The results show that the aquifer in the Sekara and Kemuning Muda is at a depth of more than 6 meters below the ground level with moderate groundwater potential. It is also found that the aquifer at depths of over 60 m have high groundwater potential. Moreover, soil porosity in Kemuning is found to be average while the ability to conduct water was moderate. This makes it possible for some surface water to seep into the soil while the remaining flows to the rivers and ditches.

In order to interpret spectral induced polarization (SIP) data measured in the field for the purpose of mineral exploration, laboratory investigations are necessary that establish relationships between electrical parameters and mineral properties. For massive ores, and in particular for seafloor massive sulfides (SMSs), samples may be difficult to obtain, and it is often not desired to cut cylindrical plugs out of the available hand pieces. We suggest a method to obtain the electrical spectra of hand pieces from measurements on the samples embedded in a non-polarizing medium, in our case quartz sand. As such, destroying potentially precious samples is not necessary. The frequency-dependent phase spectrum of the sample is calculated by dividing the bulk spectrum with a so-called dilution factor, which is obtained from numerical simulation and has a real and constant value. We evaluate the method with a set of SMS samples where conventional cylindrical plugs are available. We can estimate the phase shift maximum of 73% of the samples with a deviation less than 50% from the reference. The estimation quality slightly decreases if the dilution factor is approximated by the volumetric share of the sample. We consider the performance acceptable if the general difficulty to obtain reproducible and representative laboratory measurements for massive sulfides is taken into account.

The near-surface subsoil area undergoes active changes as a result of antropogenic impact and natural factors. Effective use of geological environment necessitates control of the area, which is complicated by high noise level. The idea of phase-measuring geoelectric control method is based on tracking the phase changes of geoelectric signals and allows to detect low-level signals indistinguishable by well-known amplitude methods. Block diagram of the two-threshold phasometric control system of the geological environment was shown, as well as its operation principles and mathematic expressions. Matlab Simulink model of the geodynamic event detector circuit was developed. Dynamic mode simulation of the model was done, and the influence of the filter's parameters on the probability of successful geodynamic detection was estimated. The obtained results confirmed the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

To optimize the rotation parameters of the drill and reduce the load on it during the drilling process, it is necessary to have information about the composition of the layers of the geological environment through which it passes. This information will reduce the wear of technical equipment and tools, as well as allow to plot optimal drilling trajectories and track the penetration of the drill into the target layer of the geological environment. This issue is especially relevant for the mining industry, where work is carried out at great depths with a complex structure of soil and rocks. Effective extraction of this information can be obtained by monitoring the phase changes of the recorded electrical signals from the receiving lines of the electrodes of the geoelectric monitoring system. The paper considers the general mechanism of implementation of the phasometric method in relation to the tasks of monitoring the drilling process, describes the equipment, as well as laboratory and field experiments to study the effectiveness of monitoring the drilling process by the phasometric method. The study of the dynamics of changes in the phase signals of the receiving lines of the electrodes during passage through the layers of soil with different electrical characteristics, which affects the change in the parameters of the recorded geoelectric signals. The research results are accompanied by graphic material confirming the possibilities of the proposed method and the prospects of its application in the extractive industry to optimize the parameters of drilling wells in real time. [ABSTRACT FROM AUTHOR]

Differences have been discovered in the deep structures of North and South Kamchatka, which raises a question about a boundary between these regions. This problem has been studied on the basis of the seismologic, inverse seismic tomographic (P-waves) and geoelectrical data obtained in recent years, as well as the information on magnetometry, gravimetry, tectonics and magmatism of the study area. Comprehensive analysis of the geological and geophysical data shows differences in the structure of the crust and upper mantle in North and South Kamchatka and common features of the deep structures of South Kamchatka and the Kuril Islands. A presumed boundary between North and South Kamchatka is related to the zone of transverse deep faults crossing the peninsula. It is evidenced by P-wave velocity anomalies at different levels of the lithosphere. This fault zone is associated with a change in the geometry and strike of the high-velocity focal layer, and reflected in the modern tectonic plan as the Petropavlovsk-Malkinsky zone of transverse dislocations. In this zone, the Avacha-Koryak group of modern volcanoes is also NW-oriented. We propose a geoelectric model showing the depths along the profile constructed across the zone. In its deep part, the model includes sub-vertical anomalies of increased electrical conductivity. The anomalies are related to deep faults. Increased electrical conductivity may be due to the presence of magmatic melts feeding volcanoes. The results obtained in our study give evidence of the common features of the deep structures of South Kamchatka and the Kuril island arc and demonstrate the differences between the deep structures of North and South Kamchatka, being separated by the zone of faults, which may penetrate into the upper mantle. It is suggested that the identified features in the South Kamchatka structure are due to deep processes taking place not only at the side of the Pacific Ocean, but also at the southern margins of the Sea of Okhotsk. These findings are of interest for geodynamics, volcanology, tectonics and other Earth sciences.

The results of a comprehensive study of electrical properties of the upper part of the Earth's crust according to the data of lateral resistivity logging of the Kola SG-3 superdeep borehole and the results of deep and superdeep sounding using controlled sources and electrical profiling on the daylight surface are presented. The possibility of extrapolation of the Kola SG-3 temperature field to a depth below the bottomhole, up to the Moho discontinuity, taking into account the data MHD sounding with the Khibiny source and with the use of industrial power transmission lines (FENICS experiment) is considered. The results of frequency electromagnetic soundings with controlled sources are analyzed, and their geodynamic interpretation is performed together with the data of drilling the SG-3 borehole. The geological and geophysical data are interpreted in the Kola SG-3 section and on the daylight surface to search for homologs and define the nature of the intermediate conductive layer of the dilatancy-diffusion nature (DD layer) discovered in the Earth's crust of the Baltic Shield at depths from 2–3 to 7–8 km. Based on the latest results of remote deep sounding (the Murman-2018 experiment), it is concluded that, in the vicinity of Kola SG-3 at a depth of 10–12 km, there is a boundary of "impenetrability" for direct current (for the galvanic mode). The geodynamic interpretation of the data is carried out taking into account the model by V.N. Nikolaevskii (1996); it has been suggested that the new boundary most closely corresponds to the brittle-to-quasi-plastic transition area of the Earth's crust. The results are compared to the log data of the SG-3 borehole and the data of superdeep drilling. [ABSTRACT FROM AUTHOR]

Post-glacial geomorphological settings bearing great amounts of colluvium as well as alluvial and moraine deposits are typical features in high mountainous environments such as the Himalayas, the Hindu Kush, the Pamirs, the Karakorum and the Tien Shan Mountains. Due to a lack of alternative living space in narrow valleys, agglomerations are often located on these unstable grounds and regularly experience dislocations across the ground surface and subsequent damage to building stock and infrastructure. Hosting two post-glacial landslide deposits and one gravitational creep within its inhabited areas, the village of Tusion, South-Western Tajikistan, was chosen to conduct geophysical surveys with the aim to determine the geomorphological causes of regularly experienced slope deformations with particular interest in the contribution of non-isolated irrigation channels crossing the mountain flanks. Over the last two decades uncontrolled seepage from a network of small-scale waterways is believed to be responsible for water infiltration to deeper horizons and - thereby - reactivate ancient sliding surfaces within the two landslide deposit zones. Also, the gravitational creep in one village part seems to be likewise triggered. Using refraction seismics and Schlumberger geoelectrics, stratigraphic structures, properties of involved rock material and saturation patterns were assessed in order to provide complementary information about the underground of concerned slopes to a previously conducted hazard assessment by separate Non-Governmental Organizations. Results of the geophysical surveys show conclusive correlations with the hazard maps and local damage reports straining that one of the major destabilizing factors for all concerned slopes consists of uncontrolled water dissipation to the underground. [ABSTRACT FROM AUTHOR]

The paper presents a shallow geoelectric field testing in Ovce Pole in the Republic of Macedonia for determining the shallow groundwaters of the given field. 11 geoelectric profiles with a Schlumberger array of the electrodes AB / 2 = 30 m were made for each geoelectric probe. The presentation of the results of the specific electrical resistance (SEO) is presented with the 1D diagrams made in IPI2win, and in the surfer, gradient horizontal maps of different depth are made. [ABSTRACT FROM AUTHOR]

We investigate how suitable ground penetrating radar (GPR) and geoelectrics are to prospect the remains of submerged wooden archaeological constructions in the water column. For this purpose, we determined the contrasts of electric resistivity and dielectric permittivity from measurements on present-day wood samples, serving as simplified approximations of water saturated and undegraded archaeological wood. As common substitutes of hard and soft construction wood, we investigated oak and spruce wood. The electric resistivity and dielectric permittivity were determined with increasing moisture content from small-scale electric and GPR measurements using a Wenner alpha array and a 2 GHz Palm antenna in a watering experiment. In a water-saturated state, resistivity values of 20 were observed. The anisotropy effects and deviations of the wood species were seen to be up to 30%. On the basis of this, the relative material contrasts of wood with respect to fresh water, sand, and clay were calculatedand compared to values found in the literature for seismic contrasts. Geoelectric, GPR, and seismic measurements show contrasts of 0.3 to 0.8, −0.4 to 0.2, and −0.24 to 0.35, depending on the surrounding material and structural orientation of the wood. The highest contrasts were found for wood in fresh water, followed by clayey and sandy subsoils. On the basis of the determined contrasts, analytical calculations were performed showing that an object of 0.5 m diameter can be detected at depths between 0.5 m and 1.5 m with geoelectrics (Schlumberger) and at depths between 0.5 m and 3 m with ground penetrating radar measurements (400 MHz).

A neural network approximation algorithm for solving inverse geoelectrics problems in the class of grid (block) models of media is presented. The algorithm is based on using neural networks for constructing an approximate inverse operator and enables formalized construction of solutions of inverse geoelectrics problem with a total number of sought-for medium parameters of ~ n · 103. The correctness of the problem of constructing neural network inverse operators is considered. A posteriori estimates of the degree of ambiguity of solutions of the resulting inverse problem are calculated. The operation of the algorithm is illustrated by examples of 2D and 3D inversions of synthetic and field geoelectric data obtained by the MTS method. [ABSTRACT FROM AUTHOR]

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At the Ketzin pilot site the GFZ German Research Centre for Geosciences has developed an interdisciplinary monitoring concept for CO 2 storage, which incorporates different permanent and periodic measurements. The concept mainly bases on seismic, geoelectric, borehole and geochemical monitoring and focusses on the storage complex, the overburden, the surface and the wellbores. After the end of injection in August 2013, this concept is now continued and applied for post-injection monitoring to address long-term site performance to finally transfer liability to the competent authority. Experiences gained at the Ketzin site may be transferred to other CO 2 storages pilots and also industrial-sized projects. [ABSTRACT FROM AUTHOR]

Tunnel excavation by means of tunnel boring machines (TBMs) is susceptible to unknown changes ahead of the tunnel face. Geophysics offers a technique called electrical resistivity that can continuously, in real-time, spatially map the formation in front of the TBM. Electrical resistivity has been successfully established for many applications including vadoze zone hydrology, oil/gas location, mineral location and failure detection in geo-structures. Yet it has not been well-established for TBM excavations. This is in part due to the complexity of the TBM tunneling environment and the uncertain influence this environment may have on the success of TBM-integrated-electrical resistivity to predict changes ahead of the tunnel face. One significant uncertainty lies in the interface region that exists around the TBM created during the modification of the virgin formation by a mechanical mixing action of the rotating cutterhead and the injection of additives used to aid in the extraction of the muck and protect the cutting tools from frictional wear. In this study, we investigate the influence of this interface region on TBM-integrated-electrical resistivity for both hard rock and soft ground tunneling conditions through finite element modeling. Regarding the performance of TBM-integrated-electrical resistivity to detect changes ahead of the cutting face, the interface region holds significant influence for both earth pressure balance (EPB) and open mode tunneling conditions. Electrical resistivity for slurry based tunneling is not influenced by the interface region. Simulations suggest that TBM-integrated-electrical resistivity can be sensitive to a formation change that is located up to five diameters in front of the TBM. [ABSTRACT FROM AUTHOR]

Strong, subcontinental mantle-dominated CO degassing occurs in the Hartoušov and Bublák mofette fields in the western Eger Rift. The combination of CO gas flux and soil gas measurements as well as gravity and geoelectric surveys provides insight into the surface and subsurface of this unique mofette area. CO soil gas and gas flux measurements reveal that large amounts of carbon dioxide are released via channels with diameters below 1 m. Carbon dioxide emissions of several tens and up to more than 100 kg day are ejected via these small seeps. Measurements with small spacings are necessary to account for the point like, focused gas discharge in the lesser degassing surrounding. We estimate that between 23 and 97 tons of CO are released over an area of about 350,000 m each day in the Hartoušov mofette field. The application of widely used geostatistical tools leads to estimations of the CO discharge with very high standard deviations due to the strong positive skewness of the data distribution. Geophysical investigations via electrical resistivity tomography and gravity measurements were carried out over areas of strong seepage and reveal distinct anomalies in the subsurface below mofettes, indicating rock and sediment alterations and/or sediment transport by pressurised, ascending CO and water mobilised by it. This study reveals that the gas emanations only occur west of a morphological step which is related to a N-S-oriented fault zone, the Počatky-Plesná fault zone. The results of CO mapping and the geophysical studies can track the course of this fault zone in this area. Our results fit into a tectonic model in which the mofette fields are in the centres of two independent pull-apart basin-like structures. We hypothesise that the sinistral strike-slip movement of the Počatky-Plesná fault zone leads to a pull-apart basin-like opening, at which the strong, mantle-derived CO degassing occurs nowadays. Since the Hartoušov and Bublák mofette fields only illustrate examples along the N-S-striking Počatky-Plesná fault zone, its role and other N-S-striking faults' roles of the Regensburg-Leipzig-Rostock zone for upper mantle degassing might have been underestimated previously. [ABSTRACT FROM AUTHOR]

ABSTRACT A large closed wire loop is generally used in field experiments for testing airborne electrical exploration equipment. Thus, methods are required for the precise calculation of an electromagnetic response in the presence of a closed wire loop. We develop a fast and precise scheme for calculating the transient response for such a closed loop laid out at the surface of a horizontally layered conductive ground. Our scheme is based on the relationship between the magnetic flux flowing through a closed loop and the current induced in it. The developed scheme is compared with 2D and 3D finite-element modelling for several positions of an airborne electromagnetic system flying over a closed loop. We also study the coupling effect between the current flowing in the closed loop and the current flowing in the horizontally layered conductive medium. The result shows that for the central position of the transmitter, the difference between axisymmetrical finite-element modelling and our scheme is less than 1%. Moreover, for the non-coaxial transmitter-receiver-loop system, the solution obtained by our scheme is in good agreement with full 3D finite-element modelling, and our total simulation time is substantially lower: 1 minute versus 120 hours. [ABSTRACT FROM AUTHOR]

Over the last decade electromagnetic (EM) measurements have provided new constraints on the upper-crustal structure of the major fault zones in the world, both when they act as conduit and as a barrier, due to strong sensitivity of resistivity to fluids circulation and mineralization. On the track of a high impact magnetotelluric (MT) study performed across the San Andreas Fault, high resolution EM data were collected in the Colfiorito epicentral area along profiles crossing some main fault lineaments. Being the study focussed both on shallow that on intermediate resistivity distribution in the brittle upper-crust, a MT profile was integrated by several electrical resistivity tomographies (ERT). The latter were successful in locating faults even where the structures are buried by a wide covering of Quaternary deposits and in the recognition of different electrical signatures of the faults. MT resistivity model crossing Mt. Prefoglio normal fault clearly imaged the typical thrust structures of the area and a high conductive zone spatially related to the fault. Seismicity seems to be located outside such conductive area, whose behaviour suggests a fluidised and altered zone incapable of supporting significant stress internally.

We study an inverse problem of object reconstruction in a layered semi-infinite domain, which arises in the mathematical modelling of electrostatic imaging methods. The reconstruction algorithm is based on a system of nonlinear integral equations of the first kind derived with the aid of a Green's function technique. The feasibility of the method is illustrated by numerical examples. [ABSTRACT FROM AUTHOR]

Abstract: Electrical conductivity measurements in the laboratory are critical for interpreting geoelectric and magnetotelluric profiles of the Earth''s crust and mantle. In order to facilitate access to the current database on electrical conductivity of geomaterials, we have developed a freely available web application (SIGMELTS) dedicated to the calculation of electrical properties. Based on a compilation of previous studies, SIGMELTS computes the electrical conductivity of silicate melts, carbonatites, minerals, fluids, and mantle materials as a function of different parameters, such as composition, temperature, pressure, water content, and oxygen fugacity. Calculations on two-phase mixtures are also implemented using existing mixing models for different geometries. An illustration of the use of SIGMELTS is provided, in which calculations are applied to the subduction zone-related volcanic zone in the Central Andes. Along with petrological considerations, field and laboratory electrical data allow discrimination between the different hypotheses regarding the formation and rise from depth of melts and fluids and quantification of their storage conditions. [Copyright &y& Elsevier]

The Trefftz method is applied to the numerical solution of the three-dimensional (3D) forward problem for an electromagnetic field harmonically oscillating with time in a 3D environment, which ensures the possibility to solve a 3D inverse problem. Here, the known idea of simultaneous joint interpretation of the tangential components of electric and magnetic fields measured on the Earth's surface, which was suggested by A.N. Tikhonov for the development of the electromagnetic sounding method, is employed in the context of the numerical-analytical representation of the forward problem solution in accordance with the Trefftz method. Application of this method allows enables one to generalize the basic plane-stratified vertically 1D model of the medium by allowing for lateral variations in electric conductivity. [ABSTRACT FROM AUTHOR]

Abstract: In this paper we present the so-called standardized pricking probe surveying technique and demonstrate its usefulness in an archaeological study. The buried target is a Palaeochristian sepulchral chapel, which had already been excavated 82 years ago, then re-buried and forgotten. By applying this technique, it was possible to locate the buried remnants of the chapel in a large field, in spite of the dense undergrowth, where classical geophysical methods could have hardly been applied. When the area was mopped-up, a detailed and systematic pricking probe surveying was carried out, and completed with geoelectric, magnetic and georadar mapping. The standardized pricking probe images, at least in this field experiment, proved to be competitive to the geophysical maps. The optimum pricking probe parameters such as horizontal interval, pricking depth, observable quantity and the way of presentation were optimized through field experiments. For a detailed investigation a rectangular grid with an interval of 50cm (i.e. a grid interval, corresponding to the wall thickness) is recommended, while for reconnaissance measurements a two times larger horizontal interval (1m in this case) proved to be sufficient. In this case study the optimum pricking depth was 20–30cm; in general it depends on the burial depth of the investigated object. For the presentation of the results a suitable running average of a two-valued observable quantity is defined. The merits of the standardized pricking probe technique are as follows: its field procedure and data processing are simple, it is cheap and relatively quick; it does not need any electronic instrument, therefore there are no investment costs and there is no risk of technical failures; the technique can be applied even among the most unfavourable field conditions like e.g. bad weather, extreme topography, dense undergrowth, etc. At the same time, the standardized pricking probe method should be applied only in areas, where the possible damaging of the buried structures is excluded. [Copyright &y& Elsevier]

The creek Jerstedter Bach is part of the Ringelheimer Mulde along the northern rim of the Harz Mountains in Northern Germany, a trough structure comprising steeply dipping limestones. The limestones are well karstified and drain parts of the region efficiently via sub-surface flow. As the entire region is intensively used by agriculture, contamination of the fast-flowing karst aquifer is a potential problem. During 2006, a small sinkhole (Windmühlenweg) developed alongside the Jerstedter Bach close to the city of Goslar. The sinkhole enlarged rapidly, with a small cave within the gravel cover diverting part of the creek to the subsurface karst catchment. Geophysical methods (gravimetry, geoelectrics) were used around the sinkhole to determine its extent and possible evolution. A negative Bouguer anomaly was found over the sinkhole, indicating more voids further downstream. The geoelectric survey helped to identify the old meandering course of the small river, which was straightened during the cultural land reform in the twentieth century. [ABSTRACT FROM AUTHOR]

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Abstract: The application of geophysical techniques to hydrogeological questions has grown in importance due to the increasing concern for environmental issues. To improve the hydrogeological image of the subsurface we carried out electric and electromagnetic measurements using Radiomagnetotellurics and Direct Current Resistivity in East-Frisia, North-West Germany. The aim of our experiments was to compare the two methods and improve the hydrogeological image of the subsurface. This allows us to assess the threat groundwater exploitation poses to wetland habitats (biotopes). Pumping groundwater from deep aquifers can drain water from near surface aquifers. Hence it is vital to know ground water confining units. Prior to our measurements the hydrogeological image of those confining units was based on interpolation between boreholes. Some single Direct Current sounding measurements were also carried out and were used with the information from the boreholes to generate a hydrogeological model of the area. By combining the results from our measurement profiles with the hydrogeological model we were able to revise the hydrogeological model. By using the geophysical modeling results we could define regions where the hydrogeological model is inadequate due to errors in interpolation. We apply both Direct Current Resistivity and Radiomagnetotellurics and perform a practical comparison of the two methods. To ensure the comparability of the resulting resistivity models we examine the depth of exploration of the two methods and demonstrate the application of the depth of exploration index to Radiomagnetotelluric data. Both methods detect conducting structures, which correlate with low permeable clay structures in the hydrogeological models. [Copyright &y& Elsevier]

The equivalence problem in geoelectrical data interpretation is a well-known fact. Generally, the equivalence analysis is implemented into 1D data inversion software but completely neglected in 2D data interpretation. In this paper, we present a practical approach to the problem of equivalence in a 2D data inversion. A 1D interpretation with equivalence analysis of the data is used to generate different starting models for a 2D inversion. At suitable locations, sounding curves are extracted from the 2D data set and inverted. Calculation of equivalent 1D results is carried out at each sounding point. The different resulting models are connected laterally to form different starting models for a 2D inversion of the complete 2D data set. Our 2D inversion results, similar in terms of data fit, but very different regarding the resulting resistivity distributions, enforce the necessity to carry out equivalence analysis in 2D data interpretations. It is shown that the implementation of a 1D interpretation of a 2D data set of predominantly layered structures provides a suitable way to introduce a first-order equivalence analysis to the problem in 2D. [Copyright &y& Elsevier]

Geoelectrical prospecting techniques have been applied in the study of the Varco d''Izzo landslide, one of the most interesting of its type. It is characterized by a rototranslational movement in the early stage and successively has undergone earthflow-type movement, which still continues today. This landslide is located in the Southern Apennine area near the city of Potenza (Italy). In particular, electrical resistivity tomographies (ERT) and self-potential surveys (SP) have been carried out by combining modern technologies for data acquisition and new methods for data inversion. The high resolution of the electrical tomographic images helped us to accurately describe the geometry of this landslide body. Joint interpretation of geophysical, geological and geomorphological data allowed us to identify the sliding surface, estimate the thickness of the mobilized material and describe the main patterns of the subsurface fluid flows in Varco d''Izzo landslide. [Copyright &y& Elsevier]